This is the 4678th most frequent Polish word.
najpiękniej
"Najpiękniej" means "most beautifully" in English.
Here, 'najpiękniej' is used as an adverb in the superlative form to describe the manner in which the song was sung, emphasizing it was done in the most beautiful way among all.
Najpiękniej zaśpiewała piosenkę finałową.
She sang the final song most beautifully.
In this sentence, 'najpiękniej' acts as an adverb modifying 'looks', indicating that the place is at its most beautiful state during the sunset.
To miejsce wygląda najpiękniej o zachodzie słońca.
This place looks the most beautiful at sunset.
Here, 'najpiękniej' is used to inquire comparatively who performs singing in the most beautiful manner out of all participants in the competition.